The invention discloses water-soluble epoxy midcoat which comprises a component A and component B. The component A comprises the following components by weight: 10-15 parts of water-soluble macromolecule amine compound, 1-5 parts of water-soluble polyamide resin, 15-20 parts of deionized water, 10-20 parts of rust resisting pigment, 25-38 parts of padding, 3-5 parts of padding, 0.1-0.5 part of dispersing agent, 0.1-0.5 part of antifoaming agent and 0.2-1 part of thickening agent. The component B comprises the following components by weight: 40-50 parts of E-51 epoxy resin, 1-6 parts of 601 epoxy resin, 2-5 parts of butyl cellosolve, 2-5 parts of non-ionic emulsifying agent, 1-5 parts of ammonium salt type or quaternary ammonium salt type emulsifying agent and 30-40 parts of deionized water. A mass ratio of the component A and the component B is 2:1. The invention further provides a preparation method of the water-soluble epoxy midcoat. With the adoption of the water-soluble epoxy midcoat, antiseptic properties and construction performance of coating can be obviously improved through matched use of two kinds of water-soluble resins, and the fact that water-soluble epoxy anticorrosive paint is used for replacing solvent type epoxy anticorrosive paint is possible.